i never said that. animals behave unpredictably all the time. lemmings run themselves off cliffs.
lets say out of all those little fishy things, 75% of them stayed in the water, 25% of them ventured onto land. onto land, where there were no predators, and no compe ion for food. do you think those 25% survived? given that they could live on land and had no predators at the time? probably. if they survived, they reproduced. their spawn lived their entire life on land. again, with no predators, no compe ion. they reproduced again. and again. and again.
if you have successful conditions, you are more likely to reproduce